(1) Field of the Invention
The invention relates to a dual-band antenna, and more particularly to a wideband design of dual-band antenna.
(2) Description of the Prior Art
Rapid innovation and development upon wireless communication technology have made mobile communication products as one of the mainstream products nowadays. These mobile communication products include mobile phones, PDAs, notebooks, etc. For sharing resources and transmitting data, they can couple with proper communication modules for linking by wiring or wirelessly with a Local Area Network (LAN) to transmit and receive e-mail and to receive instant information such as news, stocks quotations, and so on.
For instance, in atypical wireless local area network (WLAN), the operating frequency is used to locate in ISM (Industrial Scientific Medical)2.4 GHz. The ISM Band such as a blue tooth system and the group of IEEE 802 standard are widely adopted in radio communication equipments. Hence, the interference problems such as co-channel interference and/or next-channel interference are serious enough so that a trend of using another higher frequency band (say, 5.725˜5.85 GHz) has been gradually merged.
In the art, the flat inverted-F antennas have advantages of slim size and lightweight, and thus have been widely adopted as built-in antennas in most of the mobile communication products. Referring to FIG. 1 for a conventional compact printed antenna, the antenna includes a substrate 10, a ground metal 11, a strip metal 12, a short circuit leg 14 and a feeding leg 16; in which the ground metal 11, the strip metal 12, the short circuit leg 14 and the feeding leg 16 are all printed circuits located on the substrate 10. The feeding leg 16 is formed by bifurcating the metal strip 12 to reach electrically a matching circuit (not shown in the drawing). The feeding leg 16 and the ground metal 11 are not connected with each other so as to avoid a short circuit problem. The strip metal 12 is parallel with the ground metal 11. The short circuit leg 14 is located at one end (connecting a short circuit end 18) of the strip metal 12 and extends further to connect with the ground metal 11 so as to form an open circuit-short circuit structure with another end (an open circuit end 19) of the strip metal 12. The distance between the open circuit end 19 and the short circuit end 18 is preferably one quarter of the wavelength.
In order to transmit and receive signals at dual-band frequency, a constant range of one quarter of the wavelength at a lower operating frequency is the only requirement to be satisfied theoretically. In fact, the operating frequency of the aforementioned compact printed antenna is limited to single frequency band. It must be pointed out that the resonance frequency of the compact printed antenna is usually located between 8 GHz and 9 GHz which is already beyond the contemporary radio communication standard and thus will cause a serious problem in return loss Therefore, improvement upon multi-frequency operable bands is definitely needed.